4. Genetic information, variation and relationships between organisms

What is a gene?

segment of DNA that codes for a protein

what is an allele?

different forms of the same gene

what is a homologous pair of chromosomes?

A pair of chromosomes with the same genes but may have different alleles

what is a locus?

Location of a gene on a chromosome

what is the start codon?

3 bases which help initiate translation

what is the stop codon?

causes ribosomes to detach and therefore stop translation

what is the genome?

all of the DNA in an organism

what is a proteome?

The full range of proteins produced by the genome

compare DNA in a eukaryote and a prokaryote.

1. nucleotide structure is identical

2. nucleotides joined by a phosphodiester bond

3. DNA in mitochondria/chloroplasts is the same structure as DNA in prokaryotes

Contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells

1. eukaryotic DNA is longer

2. eukaryotic DNA contains introns, prokaryotic DNA does not

3. eukaryotic DNA is linear and prokaryotic DNA is circular

4. eukaryotic DNA is associated with histones whereas prokaryotic DNA is not

Compare the DNA in the nucleus and chloroplast of a eukaryote. (5 marks)

In chloroplasts

1. DNA shorter;

2. Fewer genes;

3. DNA is circular, not linear;

4. Not associated with protein/histones, unlike nuclear DNA;

5. Introns absent but present in nuclear DNA;

what does the degenerate nature of the genetic code mean?

more than one codon/triplet of bases codes for the same amino acid

what does it mean for the genetic code to be universal?

the same triplet codes for the same amino acid in all organisms

what does it mean for the genetic code to be non-overlapping?

each base is part of only one triplet

What is the advantage of the genetic code being degenerate?

even if there is a gene mutation, the triplet of bases may still code for the same amino acid

what is the advantage of the genetic code being non-overlapping?

if there is a mutation, it will only affect 1 codon and therefore only 1 amino acid. reduces the potential change in tertiary structure of a protein

what is a codon?

a triplet of bases on mRNA that codes for an amino acid

What is the difference between an exon and an intron?

Introns are the non-coding sections of DNA (do not code for a protein). Exons are the coding sections of DNA (do code for proteins).

name the 2 types of RNA involved in protein synthesis

mRNA and tRNA

mRNA is created in the ..

nucleus

what does mRNA attach to in the cytoplasm

ribosome

Why does DNA not leave the nucleus?

it is too large and would be at risk of becoming damaged

tRNA is found in the ..

cytoplasm

describe the structure of tRNA

single-stranded but folded to create a cloverleaf shape held in place by hydrogen bonds

what is the function of tRNA

it attaches to one of the 20 amino acids and transfers this amino acid to the mRNA to create the polypeptide chain

what is an anticodon?

3 bases on the tRNA which are complementary to the 3 bases on mRNA

what are the 2 enzymes involved in transcription

DNA helicase and RNA polymerase

what is the function of DNA helicase

unwinds the DNA double helix to separate the 2 strands, breaks the hydrogen bonds between DNA strands

what is the function of RNA polymerase

joins adjacent RNA nucleotides

In eukaryotic organisms, pre-mRNA is modified before leaving the nucleus. What modification is made?

the introns are spliced out by a splicesome

Why does this modification stage not occur in prokaryotic organisms?

prokaryotes do not have introns in their DNA

outline the process of transcription

1. The DNA helix unwinds to expose the bases to act as a template.

2. Only one chain of the DNA acts as a template.

3. The unwinding is catalysed by DNA helicase. DNA helicase breaks the hydrogen bonds between bases. 4. Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases.

5. The enzyme RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied.

outline the process of translation

1. mRNA attaches to a ribosome in the cytoplasm. The ribosome attaches at the start codon.

2. The tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA, which is held in place by the ribosome.

3. The ribosome will move along the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA.

4. The two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP.

5. This continues until the ribosome reaches the stop codon at the end of the mRNA molecule. The stop codon does not code for an amino acid and therefore the ribosome detaches and translation ends.

what is a gene mutation?

a change in the base sequence of the DNA

When in the cell cycle do gene mutations occur?

during DNA replication

What is a mutagenic agent?

radiation and chemicals that increase the risk of mutations

Give two reasons why gene mutations may not cause a change to the primary structure of a protein.

- the genetic code is degenerate so the new triplet of bases may still code for the same amino acid

- if the mutation occurs in an intron

Mutations in the number of chromosomes can arise spontaneously.

What is a chromosome mutation and when in the cell cycle do they occur?

Changes in the number of chromosomes

and this can arise spontaneously by chromosome non-disjunction during meiosis.

what is non-disjunction?

Non-disjunction is when the chromosomes or chromatids do not split equally during anaphase.

define polyploidy

Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two.

define aneuploidy

a change in the number of individual chromosomes

how can genetic variation be introduced

during meiosis cell division, mutations and random fertilisation

what is meiosis

a type of cell division that creates genetically different gametes resulting in 4 haploid daughter cells

genetic differences are introduced by which processes in meiosis

1. independent segregation

2. crossing over

describe how crossing over introduces variation

Crossing over occurs in meiosis I. When homologous pairs line up opposite each other at the equator in meiosis 1, parts of the chromatids can become twisted around each other. This puts tension on the chromatids, causing pairs of the chromatid to break.

The broken parts of the chromatid recombine with another chromatid (genetic recombination). This results in new combinations of alleles in the gametes.

how does independent segregation introduce variation

In meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell. It is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie. These pairs are separated, so one of each homologous pair ends up in the daughter cell.

This creates a large number of possible combinations of chromosomes in the daughter cells produced.

what is genetic diversity

the number of different alleles in a population

what is a gene pool

all the genes and alleles in a population at a particular time

what is evolution

a change in the allele frequency over many generations

what is the advantage of natural selection

results in species becoming better adapted to their environment

what are 3 key types of adaptations

1. anatomical

2. physiological

3. behavioural

describe the process of natural selection

1. Random mutations occur within the population.

2. This introduces genetic variation to the population.

3. Whilst some mutations are harmful, sometimes new alleles created by

mutations provide an organism with an advantage to survive in their

environment.

4. The new allele provides a reproductive selective advantage. Therefore

the individual with the allele is more likely to reproduce and pass the

allele onto their offspring.

5. Over many generations, there will be an increase in the frequency of this

allele within the population.

what is stabilising selection

The modal trait has a selective advantage. Occurs when there is no change in the environment.The modal trait remains the same. Standard deviation decreases, as individuals with the extreme trait decrease.

what is directional selection

One of the extreme traits has a selective advantage.Directional selection occurs when there is a change in the environment.

The modal trait changes

what is the definition of a species

a group of living organisms that can interbreed and produce fertile offspring

what is courtship behaviour and give 2 reasons it is important

The different behaviours that animals demonstrate to attract a mate. It is important for successful mating and species recognition

what is meant by a binomial system?

Individuals are named after their genus and species, eg, Homo sapiens and humans.

what is a hierarchy?

smaller groups arranged within larger groups

there are no overlaps between groups

a phylogenetic classification system arranges species into groups based on which 2 factors

1. evolutionary origins

2. evolutionary relationships

What is the order of taxa?

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

the accuracy of classification and phylogenetic trees is improving due to advances in .. ?

1. immunology sequencing

2. genome sequencing

what is a habitat

where an organism lives

what is species richness

the number of different species in a particular area at a particular time

what is the index of diversity

the relationship between the number of species in a community and the number of individuals in each species

what is the formula for the index of diversity

D = N(N-1)/sum of n(n-1)

Farming techniques can reduce biodiversity. Name farming practices which may decrease biodiversity.

1.Destruction of hedgerows

2.Selective breeding 3.Monocultures

4.Over-grazing

5.Filling in ponds and draining wetlands

Suggest 4 methods that can be used to compare genetic diversity with or between species

1.the frequency of measurable or observable characteristics

2.the base sequence of DNA

3.the base sequence of mRNA

4. the amino acid sequence of the proteins

encoded by DNA and mRNA

Why is comparing the DNA base sequence better than simply comparing observable features to determine genetic diversity? (1 mark)

more accurate measurement

Why do different species who are distantly related have very different DNA base sequences?

Mutations will accumulate in the DNA of a species overtime, creating different alleles. The longer ago a species diverged, the more time for mutations to occur and therefore the more different the DNA base sequence.

when measuring variation within a population .. are taken

samples

why do we only take samples?

because it would be too time-consuming and potentially impossible to record data on every individual within a population

in order to make sure your sample is representative of the population you must take a ... sample

large

in order to avoid bias you should .. sample

randomly